The invention relates to scanning devices. More particularly, the invention relates to devices for scanning an ultrasonic transducer across a body, in order to produce an image of a cross-section through the body. Such devices are used, for example, in medical diagnostics.
In ultrasonic "A-scanners", an ultrasonic transducer generates an acoustic pressure signal and projects the signal in a straight line through a body. The projected signal is scattered along its path of propagation, and as a result generates an echo acoustic pressure signal. The echo pressure signal contains information regarding the nature of the body along the path of propagation. The ultrasonic transducer receives the echo pressure signal, and converts it into an electric signal.
A two-dimensional image of a cross-section through the body is obtained in an ultrasonic "A-scanner", by pivoting the ultrasonic transducer through a selected angular range in order to scan the cross-sectional layer. Each electrical echo signal represents an image of a radial line in the layer; all the electrical echo signals together represent an image of a pie-shaped cross-sectional layer of the body. By suitable processing of the electrical echo signals, an image of the layer can be displayed on, for example, a cathode ray tube screen.
In practice, the ultrasonic transducer is not pivoted only one time through the selected angular range. In practice, the transducer is oscillated back and forth many times. Each repeated oscillation of the transducer produces a new image of the cross-sectional layer of the body, thus resulting in real-time imaging of the layer.
The motor used to oscillate the ultrasonic transducer must supply torque (i) to periodically reverse the direction of rotation of the transducer, (ii) to overcome frictional losses, for example due to the viscous drag of the liquid in which the transducer is typically immersed, and (iii) to cause the transducer to track a reference signal when a servo-control system is utilized. The torque required to overcome frictional losses is usually relatively small. Moreover, preferably the angular velocity of the transducer is constant throughout the scan, thereby requiring no tracking torque. Accordingly, typically 75%-90% of the torque requirement of the scanning device arises from the direction-reversal requirement.
The direction-reversal torque requirement can be explained with reference to FIG. 1, in which the angular displacement of an oscillating transducer as a function of time is shown. At each reversal of direction, the transducer is accelerated for a time, .DELTA.t, in order to reverse its direction.